Method and plant for the heat  treatment of solids containing iron oxide

ABSTRACT

A plant for the heat treatment of solids containing iron oxide. The plant includes a reactor including a fluidized bed reactor. The reactor includes a gas supply system disposed in the reactor, a stationary annular fluidized bed which at least partly surrounds the gas supply system, and a mixing chamber. The gas supply system is configured so that gas flowing through the gas supply system entrains solids from the stationary annular fluidized bed into the mixing chamber.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a divisional of application Ser. No. 10/540,435,filed on Mar. 27, 2006, which is a U.S National Phase application under35 U.S.C. §371 of International Application No. PCT/EP2003/013500, filedon Dec. 1, 2003 and which claims benefit to German Patent ApplicationNo. 102 60 733.8, filed on Dec. 23, 2002. The International Applicationwas published in English on Jul. 8, 2004 as WO 2004/057039 under PCTArticle 21(2).

FIELD

The present invention relates to a method for the heat treatment ofsolids containing iron oxide, in which fine-grained solids are heated toa temperature of 700 to 1150° C. in a fluidized bed reactor, and to acorresponding plant.

BACKGROUND

Such methods and plants are used for instance when smelting ores, forexample in the production of iron from iron ores, ferronickel alloysfrom iron-nickel ores or the like. Before heat-treated in this way theores are reduced in a succeeding process stage. While this preheating ofiron oxide containing ores previously was chiefly carried out in rotarykilns, fluidized-bed reactors have also been used for this purpose forsome years.

From EP 0 222 452 B1 there is known a method for reducing metal oxidesto obtain lower metal oxides by means of carbonaceous reducing agents,in which initially solids containing higher metal oxides are calcinedwith hot gases at 800 to 1100° C. in a first reactor in which the solidsare suspended by the hot gases. The solids calcined in this way aresubsequently reduced to form lower metal oxides in a second reactor witha stationary fluidized bed by adding carbonaceous reducing agents andoxygen-containing gases at a temperature of 800 to 1100° C. Calciningcan be carried out in a fluidized bed which is either formed stationaryor preferably circulating. However, the energy utilization of thecalcining step, which is achieved by using a stationary fluidized bed,needs improvement. This is due to the fact that the mass and heattransfer is rather moderate due to the comparatively low degree offluidization, and therefore an internal combustion is difficult tocontrol. In addition, a preheating of solids can hardly be integrated ina suspension heat exchanger, because dust-laden gases are rather notadmitted to the fluidizing nozzles of the stationary fluidized bed. Dueto the higher degree of fluidization, circulating fluidized beds on theother hand have better conditions for a mass and heat transfer and allowthe integration of a suspension heat exchanger, but are restricted interms of their solids retention time due to the higher degree offluidization.

SUMMARY

Therefore, it is the object of the present invention to improve theconditions for a mass and heat transfer during the heat treatment ofsolids containing iron oxide.

In accordance with the invention, this object is solved by a method asmentioned above, in which a first gas or gas mixture is introduced frombelow through at least one preferably centrally arranged gas supply tube(central tube) into a mixing chamber region of the reactor, the centraltube being at least partly surrounded by a stationary annular fluidizedbed which is fluidized by supplying fluidizing gas, and the gasvelocities of the first gas or gas mixture as well as of the fluidizinggas for the annular fluidized bed being adjusted such that theParticle-Froude-Numberin the central tube lie between 1 and 100, in theannular fluidized bed between 0.02 and 2, and in the mixing chamberbetween 0.3 and 30.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process diagram of a method and a plant in accordancewith a first embodiment of the present invention;

FIG. 2 shows a process diagram of a method and a plant in accordancewith a second embodiment of the present invention;

FIG. 3 shows a process diagram of a method and a plant in accordancewith a third embodiment of the present invention.

DETAILED DESCRIPTION

In the method of the invention, the advantages of a stationary fluidizedbed, such as a sufficiently long solids retention time, and theadvantages of a circulating fluidized bed, such as a good mass and heattransfer, can surprisingly be combined with each other during the heattreatment, while the disadvantages of both systems are avoided. Whenpassing through the upper region of the central tube, the first gas orgas mixture entrains solids from the annular stationary fluidized bed,which is referred to as annular fluidized bed, into the mixing chamber,so that due to the high slip velocities between the solids and the firstgas an intensively mixed suspension is formed and an optimum mass andheat transfer between the two phases is achieved. By correspondinglyadjusting the bed height in the annular fluidized bed as well as the gasvelocities of the first gas or gas mixture and the fluidizing gas, thesolids load of the suspension above the orifice region of the centraltube can be varied within wide ranges, so that the pressure loss of thefirst gas between the orifice region of the central tube and the upperoutlet of the mixing chamber can be between 1 mbar and 100 mbar. In thecase of high solids loading of the suspension in the mixing chamber, alarge part of the solids will separate out of the suspension and fallback into the annular fluidized bed. This recirculation is calledinternal solids recirculation, the stream of solids circulating in thisinternal circulation normally being significantly larger than the amountof solids supplied to the reactor from outside. The (smaller) amount ofnot precipitated solids is discharged from the mixing chamber togetherwith the first gas or gas mixture. The retention time of the solids inthe reactor can be varied within a wide range by the selection of heightand cross-sectional area of the annular fluidized bed and be adapted tothe desired heat treatment. Due to the high solids loading on the onehand and the good mass and heat transfer on the other hand, excellentconditions for a virtually complete combustion of the fuel introducedinto the reactor are obtained above the orifice region of the centraltube. There can, for instance, be performed a virtually completecombustion of natural gas close to the ignition temperature and/or withlittle excess of oxygen without local temperature peaks being obtained.The amount of solids entrained from the reactor with the gas stream iscompletely or at least partly recirculated to the reactor, with therecirculation expediently being fed into the stationary fluidized bed.The stream of solid matter thus recirculated to the annular fluidizedbed normally lies in the same order of magnitude as the stream of solidmatter supplied to the reactor from outside. Apart from the excellentutilization of energy, another advantage of the method in accordancewith the invention consists in the possibility of quickly, easily andreliably adjusting the transfer of energy and the mass transfer to therequirements by changing the flow velocities of the first gas or gasmixture and of the fluidizing gas.

To ensure a particularly effective heat transfer in the mixing chamberand a sufficient retention time in the reactor, the gas velocities ofthe first gas mixture and of the fluidizing gas are preferably adjustedfor the fluidized bed such that the dimensionlessParticle-Froude-Numbers (Fr_(P)) are 1.15 to 20 in the central tube,0.115 to 1.15 in the annular fluidized bed and/or 0.37 to 3.7 in themixing chamber. The Particle-Froude-Numbers are each defined by thefollowing equation:

${Fr}_{p} = \frac{u}{\sqrt{\frac{\left( {\rho_{s} - \rho_{f}} \right)}{\rho_{f}}*d_{p}*g}}$

with

u=effective velocity of the gas flow in m/s

ρ_(s)=density of the solid particle in kg/m³

ρ_(f)=effective density of the fluidizing gas in kg/m³

d_(p)=mean diameter in m of the particles of the reactor inventory (orthe particles formed) during operation of the reactor

g=gravitational constant in m/s².

When using this equation it should be considered that d_(p) does notindicate the grain size (d₅₀) of the material supplied to the reactor,but the mean diameter of the reactor inventory formed during theoperation of the reactor, which can differ significantly in bothdirections from the mean diameter of the material used (primaryparticles). From very fine-grained material with a mean diameter of 3 to10 μm, particles (secondary particles) with a grain size of 20 to 30 μmare for instance formed during the heat treatment. On the other hand,some materials, e.g. certain ores, are decrepitated during the heattreatment.

In accordance with a development of the invention it is proposed toadjust the bed height of solids in the reactor such that the annularfluidized bed at least partly extends beyond the upper orifice end ofthe central tube by a few centimeters, and thus solids are constantlyintroduced into the first gas or gas mixture and entrained by the gasstream to the mixing chamber located above the orifice region of thecentral tube. In this way, there is achieved a particularly high solidsloading of the suspension above the orifice region of the central tube,which allows e.g. a complete combustion under difficult conditions.

By means of the method in accordance with the invention, all kinds ofores containing iron oxide, in particular also those which contain inaddition to iron other metal oxides, can effectively be heat-treated andpossibly at the same time oxidized or reduced. In particular, the methodcan be used for the heat treatment of nickel ores containing iron oxide,manganese ores containing iron oxide and chromium ores containing ironoxide.

The generation of the amount of heat necessary for the operation of thereactor can be effected in any way known to the expert for this purpose.

In accordance with a particular embodiment of the present invention itis provided to supply fuel to the reactor, by whose combustion with anoxygen-containing gas the amount of heat required for preheating iscompletely or at least partly generated inside the reactor. In thelast-mentioned alternative, the other part of the required amount ofheat can then be covered by supplying hot gases or preheated solids.While solid fuel, such as coal, or liquid fuel, e.g. liquidhydrocarbons, is supplied to the reactor preferably via a correspondingfeed conduit directly into the annular fluidized bed or the mixingchamber, gaseous fuels, e.g. natural gas, can either be introduced via acorresponding feed conduit into the annular fluidized bed, into areactor region above the annular fluidized bed or through the centraltube into the reactor.

To ensure a complete combustion of the fuel, oxygen-containing gas withan oxygen content of 15 to 30% is preferably supplied to the reactor,namely preferably either via a conduit above the annular fluidized bedor through the central tube.

In accordance with a development of the invention it is proposed tocover part of or the entire energy demand of the reactor by supplyingexhaust gases from a downstream reactor, e.g. a reduction reactor, whichpossibly also contains fuel such as methane or carbon monoxide. Thus,the necessary demand of fresh fuel can be decreased distinctly or evenbe eliminated completely. This procedure is particularly recommendablein those methods in which after the heat treatment smelting of ironores, for instance, is performed, as large amounts of exhaust gas with atemperature of up to 1500° C. are formed thereby. Preferably, thedust-laden exhaust gas is supplied to the reactor via the central tube,so that an expensive dedusting can be omitted. The combustion air isexpediently introduced into the mixing chamber through a conduit abovethe annular fluidized bed. It is recommended to control the temperatureinside the reactor by varying the amount of air supplied, the gasatmosphere at the outlet of the reactor still being slightly reducing.

When the calorific value of the exhaust gas of the reduction reactor isnot sufficient for reaching the desired reactor temperature, it turnedout to be advantageous to supply a mixture of an oxygen-containing gas,of gaseous fuel such as natural gas, and of exhaust gas from thedownstream second reactor, which likewise contains fuel, to the reactorthrough the central tube. With this procedure, the mixing of the streamspreferably takes place in the central tube, whereas ignition andcombustion are effected in the mixing chamber, where a particularlyeffective heat transfer takes place between the hot particles of thestationary annular fluidized bed, which were entrained by the gasstream, and the process gases. In this case, the reactor temperature iscontrolled by varying the flow rate of the gaseous fuel, the amount ofthe oxygen-containing gas being adjusted such that a residual oxygencontent of the exhaust gas is still present at the outlet of thereactor.

In accordance with another embodiment of the present invention, freshfuel, preferably gaseous fuel, or fuel-containing exhaust gas from adownstream reactor or a mixture of fresh fuel and fuel-containingexhaust gases together with oxygen-containing gas is burnt in acombustion chamber upstream of the reactor, before the hot process gasesthus generated are supplied to the reactor, preferably via the centraltube. In this embodiment it is of course also possible to generate onlypart of the energy demand by the combustion of fresh fuel and cover theremaining part by supplying hot exhaust gases from a downstream reactor.

When the reactor is operated with high pressure, the reactor pressurecan be utilized by using an expansion turbine. The preferred pressurevalues would be between 0.8 and 10 bar.

As gas for fluidizing the annular fluidized bed, dust-free hot or coldair is preferably supplied to the preheating reactor, and for thispurpose, all other dust free gases or gas mixtures known to the expertfor this purpose can of course also be used. It may also be advantageousto compress dedusted and cooled exhaust gas such that it can be utilizedas fluidizing gas for the annular fluidized bed.

The amount of solids which is entrained by the gas stream flowingthrough the central tube and is discharged from the reactor, i.e. thatamount which in the mixing chamber of the reactor does not fall backinto the stationary annular fluidized bed, is separated in a cyclonedownstream of the reactor and can completely or partly be recirculatedvia a solids return conduit. An essential advantage of this solidsrecirculation consists in that the solids loading of the suspension inthe mixing chamber can specifically be adjusted to the requirements ofthe process, and even be changed during the operation as required.

In accordance with a development of this invention, the pressure lossbetween the central tube and the discharge conduit from the reactor ismeasured for this purpose and controlled by varying the amount of solidsrecirculated. It turned out to be particularly advantageous that afluidized intermediate container with downstream dosing device, forinstance a variable-speed rotary-vane (star) feeder or a roller-typerotary valve. The solids not needed for recirculation are dischargede.g. by means of an overflow.

When influencing the solids load of the suspension above the orificeregion of the central tube is not required or a recirculation is notexpedient for other reasons, the solids recirculation and theintermediate container can be omitted. The solids discharged with thegas stream are discharged completely in this case.

Upstream of the reactor, one or more preheating stages may be provided,in which the ore to be calcined and possibly to be reduced is preheated,and thus part of its moisture content is removed. Preferably, twopreheating stages, each consisting of a suspension heat exchanger and adownstream cyclone, are provided upstream of the reactor, the materialin the first suspension heat exchanger being heated by exhaust gas fromthe second suspension heat exchanger, and the material in the secondsuspension heat exchanger being heated by exhaust gas from the reactor.In this way, the total energy demand of the process is reduced.

In accordance with a development of the invention it is furthermoreproposed to directly introduce into the reactor a part (0 to 100%) ofthe solids separated in the cyclone of the first preheating stage via abypass conduit bypassing the second preheating stage, in dependence onthe moisture content of the starting material, whereas the remainingamount is first introduced into the second preheating stage, before thesame is also introduced into the reactor. The higher the moisturecontent of the starting material to be preheated and possibly to bereduced, the smaller will be chosen the amount of solids passed throughthe second preheating stage and the larger will be chosen the amount ofsolids passed through the bypass conduit. Thus, the procedure canflexibly be adjusted to the moisture content of the starting materialwith regard to an optimum utilization of energy.

A plant in accordance with the invention, which is in particular suitedfor performing the method described above, has a reactor constituting afluidized-bed reactor for preheating and/or oxidizing or (pre.)reducingsolids containing iron oxide, the reactor having a gas supply systemwhich is formed such that gas flowing through the gas supply systementrains solids from a stationary annular fluidized bed, which at leastpartly surrounds the gas supply system, into the mixing chamber.Preferably, this gas supply system extends into the mixing chamber. Itis, however, also possible to let the gas supply system end below thesurface of the annular fluidized bed. The gas is then introduced intothe annular fluidized bed e.g. via lateral apertures, entraining solidsfrom the annular fluidized bed into the mixing chamber due to its flowvelocity.

In accordance with a preferred aspect of the invention, the gas supplysystem has a gas supply tube (central tube) extending upwardssubstantially vertically from the lower region of the reactor preferablyinto the mixing chamber, which is at least partly surrounded by achamber in which the stationary annular fluidized bed is formed. Thecentral tube can constitute a nozzle at its outlet opening and have oneor more apertures distributed around its shell surface, so that duringthe operation of the reactor solids constantly get into the central tubethrough the apertures and are entrained by the first gas or gas mixturethrough the central tube into the mixing chamber. Of course, two or morecentral tubes with different or identical dimensions or cross-sectionalshapes may also be provided in the reactor. Preferably, however, atleast one of the central tubes is arranged approximately centrally withreference to the cross-sectional area of the reactor.

In accordance with a preferred embodiment, a cyclone for separatingsolids is provided downstream of the reactor, where the cyclone can havea solids conduit leading to the annular fluidized bed of the firstreactor.

To provide for a reliable fluidization of the solids and the formationof a stationary fluidized bed, a gas distributor is provided in theannular chamber of the reactor, which divides the chamber into an upperfluidized bed region and a lower gas distributor chamber. The gasdistributor chamber is connected with a supply conduit for fluidizinggas. Instead of the gas distributor chamber, there can also be used agas distributor composed of tubes.

For adjusting the temperatures necessary for preheating the solids, thereactor preferably has a fuel supply conduit leading to the centraltube, the annular chamber and/or the mixing chamber. For the samepurpose, a supply conduit for oxygen-containing gas is provided in thereactor, which either leads to the central tube or into a region abovethe fluidized bed region.

In addition or alternatively, a combustion chamber may be providedupstream of the reactor, in which fresh fuel and/or fuel-containingexhaust gases from a reactor downstream of the preheating reactor areburnt.

In accordance with a development of the invention, it is proposed toprovide a gas conduit leading from a reduction reactor downstream of thepreheating reactor to the central tube of the reactor, through which gasconduit at least part of the exhaust gases of the reduction reactor canbe supplied to the preheating reactor.

Since extreme temperatures can be generated thereby for lack of solids,which extreme temperatures can for instance result in high NO_(x)emissions or material problems, an internal combustion is preferred ingeneral.

In the annular fluidized bed and/or the mixing chamber of the reactor,means for deflecting the solid and/or fluid flows may be provided inaccordance with the invention. It is for instance possible to positionan annular weir, whose diameter lies between that of the central tubeand that of the reactor wall, in the annular fluidized bed such that theupper edge of the weir protrudes beyond the solids level obtained duringoperation, whereas the lower edge of the weir is arranged at a distancefrom the gas distributor or the like. Thus, solids raining out of themixing chamber in the vicinity of the reactor wall must first pass bythe weir at the lower edge thereof, before they can be entrained by thegas flow of the central tube back into the mixing chamber. In this way,an exchange of solids is enforced in the annular fluidized bed, so thata more uniform retention time of the solids in the annular fluidized bedis obtained.

The invention will subsequently be described in detail with reference toembodiments and the drawing. All features described and/or illustratedin the drawing form the subject-matter of the invention per se or in anycombination, independent of their inclusion in the claims or theirback-reference.

In the method shown in FIG. 1, which is in particular suited forpreheating and prereducing iron-nickel ores and iron-manganese ores,fine-grained, possibly moist ore with a grain size of less than 10 mm ischarged via a screw conveyor 1 into a suspension heat exchanger 2 of afirst preheating stage, in which the material is preferably suspendedand preheated by exhaust gas from a second preheating stage, until alarge part of the surface moisture of the ore has been removed.Subsequently, the suspension is conveyed by the gas stream into acyclone 3, in which the solids are separated from the gas. The separatedsolids then are conveyed through a conduit 4 into a second Venturi-typesuspension heat exchanger 5, heated up further and again separated fromthe gas stream in a cyclone 6.

The ore thus preheated is conveyed through conduit 7 into the reactor 8,in which the material is heated to temperatures of 700 to 1150° C. forremoving the residual crystal water. In its lower central region, thereactor has a vertical central tube 9 which is surrounded by a chamberof annular cross-section. Both the central tube 9 and the “annularchamber” can of course also have a cross-section different from thepreferred round cross-section, as long as the annular chamber at leastpartly surrounds the central tube 9.

The annular chamber is divided into an upper and a lower part by a gasdistributor 11. While the lower chamber serves as gas distributorchamber (wind box) 10 for fluidizing gas, the upper part of the chamberincludes a stationary fluidized bed 12 (annular fluidized bed) offluidized ore, e.g. iron ore, or a nickel, chromium or manganese orecontaining iron oxide, the fluidized bed extending slightly beyond theupper orifice end of the central tube 9.

Through conduit 13, air is supplied to the reactor as fluidizing gaswhich flows through the gas distributor 11 into the upper part of theannular chamber, where it fluidizes the ore to be heated by forming astationary fluidized bed. The velocity of the gases supplied to thereactor 8 preferably is chosen such that the Particle-Froude-Number inthe annular fluidized bed 12 lies between 0.12 and 1.

Through the central tube 9, exhaust gas from a downstream reductionreactor 14 can constantly be supplied to the reactor 8, which afterpassing through the central tube 9 said exhaust gas flows through amixing chamber 15 and an upper passage 16 into the cyclone 17. Thevelocity of the gas supplied to the reactor 8 preferably is adjustedsuch that the Particle-Froude-Number in the central tube 9 lies between6 and 10. Due to these high gas velocities, the gas flowing through thecentral tube 9 entrains solids from the stationary annular fluidized bed12 into the mixing chamber 15 when passing through the upper orificeregion. Due to the banking of the fluidized bed in the annular fluidizedbed as compared to the upper edge of the central tube 9, the fluidizedbed flows over this edge towards the central tube 9, whereby anintensively mixed suspension is formed. The upper edge of the centraltube 9 may be straight or indented or have lateral inlet openings. As aresult of the reduction of the flow velocity by the expansion of the gasjet and/or by impingement on one of the reactor walls, the entrainedsolids quickly lose speed and fall back again into the annular fluidizedbed 12. Only a small part of non-precipitated solids is entrained fromthe reactor together with the gas stream via the transition duct 16.Between the reactor regions of the stationary annular fluidized bed 12and the mixing chamber 15 there is thus obtained a solids circulationwhich ensures a good heat transfer. Solids separated in the cyclone 17are recirculated to the reactor 8 via the conduit 18, while the stillhot exhaust gas is introduced into the suspension heat exchanger 5 ofthe second preheating stage.

The required process heat is covered by the combustion of fuel. For thispurpose, e.g. natural gas is supplied to the reactor as fuel, which viaconduit 19 is first introduced into conduit 20 and then via the centraltube 9 into the reactor 8. Alternatively or in addition, solid fuel suchas coal can also directly be introduced into the annular fluidized bed12. Liquid fuels are expediently atomized with a gas in a two-fluidnozzle. The atomizing gas also cools the nozzle.

Another possibility is the fluidization of the annular fluidized bed 12with gaseous fuel or a fuel-containing gas mixture. If no fuel isrequired, the gas distribution chamber must, however, be flushed withinert gas, e.g. nitrogen, to be able to switch over to air fluidization.This turned out to be expedient, in order to avoid an interruption ofthe fluidization of the annular fluidized bed 12.

In a non-illustrated further embodiment of a tubular gas distributor, agas distribution chamber is omitted. The annular fluidized bed 12 isfluidized by air which is introduced through nozzles. The air issupplied to the nozzles by means of a manifold. Individual nozzles maybe connected to a fuel supply conduit, so that fuel can be introduced.In this embodiment, the fluidization of the annular fluidized bed by airis maintained, even if no or little fuel is required.

In the preferred circuit as shown in FIG. 1, a fuel-containing exhaustgas of the downstream reduction reactor 14 is supplied through conduit20. The energy content of this exhaust gas preferably is sufficient toachieve the desired reactor temperature. To ensure a complete combustionof the fuel, an oxygen-containing gas, preferably with an oxygen contentof 15 to 30 vol-%, is supplied to the reactor, the gas first beingintroduced via the supply conduit 21 into the conduit 20 leading to thecentral tube 9, before flowing into the reactor 8 via the central tube9. In the central tube 9, a mixture of fuel-containing exhaust gas andoxygen-containing exhaust gas should be obtained, whereas ignition andcombustion should only take place in the reactor 8. Alternatively, theoxygen-containing gas can also be introduced into the reactor 8 via asupply conduit above the annular fluidized bed 12.

A particular advantage of the method of the invention consists in thatthe exhaust gas from the downstream reduction reactor 14, which has beenintroduced via the central tube 9 and contains gaseous fuel such asmethane and carbon monoxide, can also be burnt in the reactor 8 and thusbe utilized energetically without first having to be dedusted.

From the annular fluidized bed 12, part of the preheated material iscontinuously withdrawn from the reactor 8 via conduit 22 and introducedinto the fluidized bed of the reduction reactor 14, in which the metaloxides contained in the solids are reduced to obtain lower metal oxidesand/or metals. For the same purpose, preheated solids, which wereseparated in an electrostatic precipitator 23 from the exhaust gas ofthe cyclone 3 downstream of the first suspension heat exchanger 2, aresupplied to the reduction reactor 14 via a conduit 24. As reducingagent, for instance reduction gas recovered from natural gas in anupstream cracking plant is used. This reduction gas is supplied to thereactor 14 via a conduit 25 through a tuyere bottom or gas distributor33. In the case of a smelting reactor (cf. FIG. 3), coal dust can beinjected into the smelting reactor as reducing agent.

Alternatively or in addition, liquid hydrocarbons or fine-grained coalcan also be used as reducing agent, which can either be directlyintroduced into the stationary fluidized bed of the reactor 14 or besupplied to the reactor 14 together with the preheated or calcinedsolids via the conduits 26, 22. If liquid or solid reducing agents areused, an oxygen-containing fluidizing gas with an oxygen content of 10to 25 vol-% must in addition be supplied to the reduction reactor viaconduit 25 for forming the stationary fluidized bed. Reduced solidsleave the reduction reactor 14 via conduit 27, while the dust-ladenexhaust gas is supplied to the reactor 8 via conduit 20 and the centraltube 9 without separating the dust content, in which reactor the fuelstill contained in the exhaust gases is burnt. In this way, the exhaustgas from the reduction reactor 14 is utilized on the one hand as fuelfor generating the temperature required in the reactor and on the otherhand as carrier gas for suspending the solids entrained from the orificeregion of the central tube 9 in the mixing chamber 15. Due to theenergetic utilization of the exhaust gas from the reduction reactor 14in the reactor 8 on the one hand and the optimum utilization of energyduring preheating on the other hand, which is achieved as a result ofthe design of the reactor 8, a high efficiency is achieved by means ofthe method of the invention.

To obtain a greater flexibility as regards the choice of the startingmaterials, in particular with regard to the moisture of the ore used,with a chosen dimensioning of the reactor 8, there is provided a bypassconduit 28 leading from the cyclone 3 of the first preheating stage tothe reactor 8, through which bypass conduit a predetermined amount ofthe solids separated in the cyclone 3 is directly introduced into thereactor 8. The remaining amount of solids is first passed through thesecond preheating stage, before the same is also introduced into thereactor 8 via conduit 7. In the case of particularly moist ores, thebypass conduit 28 allows to pass only a small partial stream through thesecond preheating stage or switch the same off completely, in order toavoid the condensation of steam in the electrostatic precipitator 23.

In accordance with the invention, the exhaust gas temperature is keptconstant, in order to maximize the utilization of energy and avoid acondensation and thus corrosion damages in the exhaust gas path. Thecontrol of the exhaust gas temperature is effected in that in the caseof high moisture and a decrease of the exhaust gas temperature in thecyclone 3 below the desired value, the feed rate of a metering device,for instance the rotational speed of a rotary vane feeder 34 or thelike, is increased in the bypass conduit 28 (TIC1). As a result, morecold solids enter the reactor 8 and the temperature in the passage 16falls below the desired value. By means of a further temperature control(TIC2) this leads to a greater opening of a fuel valve 19′ in the fuelconduit 19. At the same time, less cold solids from the cyclone 3 getinto the heat exchanger 5, so that the temperature in the heat exchanger5 and the cyclone 6 rises in the direction of the desired value.

In contrast to the apparatus described above, the plant shown in FIG. 2has a combustion chamber 29 upstream of the reactor 8, in which fuel orfuel-containing exhaust gas from a downstream melting reactor 30 isburnt before being introduced into the reactor 8.

To the combustion chamber 29, fuel-containing exhaust gas from themelting reactor 30 is supplied via conduit 19, air preheated in a heatexchanger 31 is supplied as combustion gas via conduit 21, and likewisepreheated low-oxygen recycle gas is supplied via conduit 32. From thecombustion chamber 29, the hot process gas generated by combustion,which has a temperature between 900 and 1700° C., is withdrawn viaconduit 20 and introduced into the reactor 8 via the central tube 9,where the process gas fluidizes and preheats the solids introduced intothe annular fluidized bed 12 via conduit 7. Furthermore, fluidizing gasfor the annular fluidized bed 12 is supplied to the reactor 8 viaconduit 13, and tertiary air for the temperature and oxygen control issupplied to the reactor 8 via conduit 35. Preferably, the velocities ofthe fluidizing gas and of the gas flowing through the central tube 9 arechosen such that the Particle-Froude-Numbers in the annular fluidizedbed 12 lie between 0.12 and 1 and in the central tuyere 9 between 6 and12.

Gas/solids mixture discharged from the reactor 8 is separated into thetwo phases in the cyclone 17. While the preheated solids are introducedinto the smelting reactor 30 via conduit 22, the warm exhaust gas isfirst passed through the heat exchanger 31 and then cleaned by anon-illustrated gas cleaning device.

By means of this method it is ensured that the fuel is burnt completelybefore it is introduced into the reactor 8.

The method illustrated in FIG. 3 differs from the one described in FIG.1 in that the energy demand of the reactor 8 is exclusively covered bysupplying hot exhaust gas from a downstream smelt reduction reactor 14′.Such reactors 14′ are used for instance for the melt reduction of ironore to obtain metallic iron, where considerable amounts of dust-ladenexhaust gases having a temperature of about 1500° C. are produced.

Analogous to the method illustrated in FIG. 1, iron ore is firstpreheated in two preheating stages, each consisting of a suspension heatexchanger 2, 5 and a downstream cyclone 3, 6, before the solids areintroduced into the annular fluidized bed 12 of the reactor 8 viaconduit 7.

To the reactor 8, air is supplied as fluidizing gas via conduit 13 andexhaust gas of the downstream melt reduction reactor 14′ is supplied viathe central tube 9. Air is introduced via the gas stream conduit 19.Since the dust-laden exhaust gas is supplied to the reactor 8, anexpensive dedusting can be omitted. Preferably, the velocities of thefluidizing gas and the gas flowing through the central tube 9 are chosensuch that the Particle-Froude-Numbers in the annular fluidized bed 12are between 0.1 and 1, in the central tuyere 9 between 5 and 10, and inthe mixing chamber 15 between 1 and 5.

A partial stream of the heat-treated solids separated in the cyclone 17is recirculated to the reactor 8 via conduit 18, whereas the otherpartial stream is supplied to the reactor 14′ via conduit 22 for meltreduction.

A particular advantage of this method as compared to the methods knownso far for this purpose consists in that there can be omitted anexpensive dedusting of the exhaust gas from the melt reduction reactor14′, which is absolutely necessary before introducing the exhaust gasinto a classical stationary fluidized bed. Since, moreover, in thismethod the supply of additional fuel can be omitted, an even betterutilization of energy is obtained as compared to the method shown inFIG. 1.

The invention will be described below with reference to three examplesdemonstrating the inventive idea, but not restricting the same.

Example 1 Heat Treatment of Lateritic Nickel Ore

In a plant corresponding to FIG. 1, 220 t/h lateritic nickel ore with agrain size of less than 10 mm, containing

1.75 wt-% NiO, 31.4 wt-% Fe₂O₃,   11 wt-% moisture,

were supplied to the suspension heat exchanger 2 by means of the screwconveyor.

Upon passage through the first and second preheating stages, thepredried nickel ore was introduced into a calcining reactor 8 viaconduit 7. Furthermore, 6,200 Nm³/h natural gas as fuel (through conduit19), 71,000 Nm³/h air as combustion gas (through conduit 21) as well as32,600 Nm³/h exhaust gas from the reduction reactor (through conduit 20)were supplied to the calcining reactor 8 via the central tube 9, the gashaving a temperature of about 800° C. and the following composition:

 2 vol-% H₂ 18 vol-% H₂O 10 vol-% CO 14 vol-% CO₂  1 vol-% CH₄ 44 vol-%N₂.

In addition, 15,000 Nm³/h air were supplied to the reactor via conduit13 as fluidizing gas for forming the annular fluidized bed 12. Thetemperature in the calcining reactor 8 was 900° C.

From the calcining reactor, 173 t/h calcined material were withdrawn,and the same amount was supplied to the reduction reactor 14 via conduit22. Furthermore, 32,600 Nm³/h reduction gas, which also served asfluidizing gas, were supplied to the reduction reactor via conduit 25,the reduction gas having the following composition:

30 vol-% H₂ 25 vol-% CO  1 vol-% CH₄ 44 vol-% N₂.

Finally, 27,168 t/h calcined and prereduced solids (nickel ore) werewithdrawn from the reduction reactor via conduit 27, which solidscontained 1.6 wt-% metallic nickel and 35.5 wt-% FeO.

Example 2 Heat Treatment of Chromium-Containing Iron Ore

In a plant corresponding to FIG. 2, 30 t/h chromium ore containing ironoxide with a moisture content of 5 wt-%, a Cr₂O₃ content of 53 wt-% anda grain size of not more than 6 mm were supplied to the reactor 8through conduit 7.

To the combustion chamber 29, 4,500 Nm³/h fuel gas were supplied throughconduit 19, 5,800 Nm³/h air preheated to 450° C. were supplied throughconduit 21′, and 4480 Nm³/h recycle gas likewise preheated to 450° C.were supplied through conduit 32. At the opposite side of the combustionchamber, 13,600 Nm³/h of hot process gas generated by combustion, whichhad a temperature of about 1600° C., were withdrawn through conduit 20and supplied to the reactor via the central tube 9. Furthermore, 7,100Nm³/h air were fed into the reactor as fluidizing gas via conduit 13.

21,300 Nm³/h exhaust gas with a temperature of 1100° C. were withdrawnfrom the cyclone 17, cooled to 870° C. in the succeeding heat exchanger31, and ultimately cleaned in a gas cleaning device. Finally, 28.4 t/hchromium-containing ore with a temperature of 1100° C. were withdrawnfrom the calcining reactor via conduit 22 and supplied to the meltingreactor 30.

Example 3 Heat Treatment of Iron Ore

In a plant corresponding to FIG. 3, 178 t/h moist iron ore (hematite)with a moisture content of 5 wt-%, an Fe₂O₃ content of 80 wt-%, and agrain size of less than 10 mm were supplied to the suspension heatexchanger 2 via the screw conveyor 1 and dried with exhaust gas from thecyclone 6 and preheated to about 277° C. The exhaust gas from thecyclone 6 had the following composition:

46.9 vol-% N₂  7.6 vol-% H₂ 11.4 vol-% H₂O  5.7 vol-% CO 28.4 vol-% CO₂.

Subsequently, the solids were separated from the gas phase in thecyclone 3 and transferred to the suspension heat exchanger 5, in whichthey were further heated to a temperature of 561° C. by contact with hotexhaust gas of about 850° C. from the cyclone 17. Thereupon, thematerial was passed through the cyclone 6 and conduit 7 into the annularfluidized bed 12 of the reactor 8.

Via the central tube 9, a mixture of 13,000 Nm³/h air (conduit 19) and103,000 Nm³/h hot exhaust gas of about 1000° C. (conduit 20) from themelt reduction reactor 14′ was supplied to the reactor with a flowvelocity of 65 m/s. The exhaust gas had the following composition:

45.1 vol-% N₂ 5.2 vol-% H₂ 8.7 vol-% H₂O 18.5 vol-% CO 22.5 vol-% CO₂20-40 g/Nm³ dust.

In addition, about 20,000 Nm³/h air were supplied to the reactor viaconduit 13 as fluidizing gas for forming the annular fluidized bed.

A partial combustion of the exhaust gas from the melt reduction reactor14′ with the air supplied at the same time took place in the lowerregion of the reactor. Due to the reducing gas atmosphere in the reactor8, part of the hematite was prereduced to produce magnetite (Fe₃O₄).

LIST OF REFERENCE NUMERALS

1 screw conveyor

2 suspension heat exchanger of the first preheating stage

3 cyclone of the first preheating stage

4 solids conduit

5 suspension heat exchanger of the second preheating stage

6 cyclone of the second preheating stage

7 solids conduit

8 reactor

9 central tube

10 gas distributor chamber (wind box)

11 gas distributor

12 annular fluidized bed

13 supply conduit for fluidizing gas

14,14′ reduction reactor

15 mixing chamber

16 transition duct

17 cyclone

18 solids return conduit

19,20,21 gas stream conduit

22. supply conduit for heat-treated solids

23 electrostatic precipitator

24 solids supply conduit

25 feed conduit for fluidizing gas/gaseous reducing agent

26 supply conduit for solid reducing agent

27 product discharge conduit

28 bypass conduit

29 combustion chamber

30 melting reactor

31 heat exchanger

32 recycle gas conduit

33 gas distributor

34 star feeder

35 tertiary air conduit

1-28. (canceled) 29: A plant for the heat treatment of solids containingiron oxide, the plant comprising: a reactor including a fluidized bedreactor comprising; a gas supply system disposed in the reactor; astationary annular fluidized bed which at least partly surrounds the gassupply system; and a mixing chamber, wherein the gas supply system isconfigured so that gas flowing through the gas supply system entrainssolids from the stationary annular fluidized bed into the mixingchamber. 30: The plant recited in claim 29, wherein the gas supplysystem comprises at least one gas supply tube extending upwardssubstantially vertically from a lower region of the reactor into themixing chamber of the reactor, the gas supply tube being at least partlysurrounded by a chamber in which the stationary annular fluidized bed isformed. 31: The plant recited in claim 29, wherein the gas supply tubeis disposed approximately centrally with reference to a cross-sectionalarea of the reactor. 32: The plant recited in claim 29, wherein a shellsurface of the gas supply tube includes openings. 33: The plant recitedin claim 32, wherein the openings of the gas supply tube are slots. 34:The plant recited in claim 29, further comprising a cyclone disposeddownstream of the reactor, the cyclone being configured to separatesolids and including a solids conduit leading to the annular fluidizedbed of the reactor. 35: The plant recited in claim 29, wherein thereactor further includes an annular chamber with a gas distributor, thegas distributor dividing the chamber into an upper fluidized bed regionand a lower gas distributor chamber, the lower gas distributor chamberbeing connected with a supply conduit configured to supply fluidizinggas to the lower gas distributor chamber. 36: The plant recited in claim35, wherein the reactor further comprises at least one of a fuel supplyconduit leading to the gas supply tube and a fuel supply conduit leadingto the annular chamber. 37: The plant recited in claim 29, wherein thereactor further comprises an oxygen supply conduit for anoxygen-containing gas, the oxygen supply conduit leading to the gassupply tube or into a region above the annular fluidized bed. 38: Theplant recited in claim 29, further comprising a combustion chamberdisposed upstream of the reactor. 39: The plant recited in claim 29,wherein the gas supply tube of the reactor is connected with a secondreactor disposed downstream of the reactor via a supply conduit.